Changes global allocator to mimalloc, improving performance. (#9415)

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this PR makes nushell use mimalloc as the default allocator, this has
the benefit of reducing startup time on my machine. `17%` on linux and
`22%` on windows, when testing using hyperfine.
the overhead to compile seem to be quite small, aswell as the increase
of binary size quite small
on linux the binary went from `33.1mb` to `33.2mb`

linux

![image](https://github.com/nushell/nushell/assets/17986183/ba5379b4-2c08-483a-a9ff-a9d8524d2943)

windows 

![image](https://github.com/nushell/nushell/assets/17986183/fda5090f-96a9-48d1-ada4-617694b9d880)


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src/main.rs

@@ -9,6 +9,10 @@ mod test_bins;
 #[cfg(test)]
 mod tests;
 
+#[cfg(feature = "mimalloc")]
+#[global_allocator]
+static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc;
+
 use crate::{
     command::parse_commandline_args,
     config_files::set_config_path,